This invention relates to micromachined flywheels and/or reaction wheels, and particularly to micromachined flywheels and/or reaction wheels used for energy storage and attitude control in a lightweight space vehicle.
Possibilities for lightweight satellites are currently being explored. It is hoped that such satellites will eventually have mass reduced by an order of magnitude as compared to conventional satellites. Lightweight satellites present a new paradigm of spacecraft that calls for performing the same missions as today""s satellites, but at lower cost. For example, one goal is to have a 100 kg satellite of the future perform the same work as today""s 1000 kg satellite. Another goal calls for satellites having mass of about 10 kg working in reconfigurable clusters for space surveillance and other purposes. In addition, new logistic space missions will likely include, for example, on-orbit servicing, and de-orbiting. Such new logistic missions will also need small, very agile satellites with long lifetimes.
Planned lightweight satellites are being called micro-satellites and nano-satellites. It is expected that some form of micro-satellites and nano-satellites will be developed for flight demonstration in the next two to three years. These new satellites will require stored energy for power, and attitude control devices for maintaining or changing orbits.
Attitude control devices that are expected to be available for micro-satellites and nano-satellites include gravity booms or tethers, and microthrusters. Unfortunately, gravity booms and tethers are not weight efficient. This is true even though tethers also generate power. As a further drawback, tethered satellites may de-orbit in about four months because of drag induced by the energy extraction. Most significantly, booms and tethers are not considered suitable for agile satellites that may be deployed for logistic missions. One reason that such devices are not desirable is that booms and tethers are slow when adjusting satellite position to precisely point an antenna or camera. Further, such systems are restricted to pointing in only one direction. Microthrusters can provide mobility but have limited fuel capacity.
Another important requirement for the planned micro-satellites and nano-satellites is energy storage. Energy storage on satellites requires batteries that are light, small and rechargeable over many cycles. It is expected that solar panel powered satellites will need up to about a 35,000 charge/discharge cycle capability. Batteries that meet these requirements are not available. Known rechargeable batteries, e.g., nickel hydrogen (Ni H) or lithium ion (Li Ion) batteries, cannot be recycled more than 1000 times. Moreover, such batteries require stable room temperatures for high energy capacity and long life. Temperature stabilization, in turn, requires adding temperature stabilization circuits that increases system weight and cost. The energy density of Ni H batteries at system level is 25 Whr/kg. Li Ion batteries may have an energy density number as high as 50 Whr/kg. The need for power conversion and temperature stabilization circuits lowers the energy density number significantly from the battery only power density. The microwheel used as a flywheel may achieve energy density of 50 Whr/kg and it can be recharged 100,000 times or more. It also may provide high surges of current which are not available from batteries.
It has been shown above that energy storage and attitude control requirements for lightweight satellites have not been adequately addressed by presently known systems. For both applications low cost and low losses are desirable.
The present invention provides for the first time a rotating microwheel for meeting energy storage and attitude control requirements for lightweight satellites. In order to meet these goals the present invention offers an innovative and affordable micromachined reaction/flywheel that integrates the attitude control and energy storage and electronics functions. The microwheel allows pointing with high precision, analogously to large satellites that use much larger mechanical momentum and gyroscopic wheels. The new microwheel constructed in accordance with the invention, is highly modular so it can serve micro-satellites and nano-satellites with different levels of redundancy.
One object of the invention is to provide an attitude control reduced in mass by factor of about 10 as compared to the state of the art reaction wheels to enable existing missions with lower weight and open possibilities for new missions.
It is another object of the invention to provide a micro-machined wheel, or microwheel, that complements microthrusters resulting in a simpler, lighter, structure of a lightweight microsatellite that uses fuel more efficiently when the attitude is controlled by the microwheel used as a momentum transfer device.
It is another object of the invention to provide micromachined wheels that can be easily manufactured in modules of 5 Nms momentum and 40 W/hr energy capacity in order to provide attitude control, which is essential for cluster control, cheaply and at low weight for satellites having mass of about 10 kg working in reconfigurable clusters for space surveillance and other missions.
It is another object of the invention to provide a highly integrated micromachined flywheel for small, very agile satellites having long lifetimes to enable space missions such as, for example, on-orbit servicing, and de-orbiting.
It is another object of the invention to provide a highly integrated micromachined flywheel for multifunction integration in a lightweight satellite for cost, weight, and power reduction.
It is another object of the invention to provide a microwheel comprising silicon crystal material having low cost (e.g. $40/wafer) and high rupture modulus allowing high rotational speed approaching 6000 rotations per second, that is the same as the strongest carbon matrix materials.
It is another object of the invention to provide a microwheel having high precision, micromachined flatness and dimensions for small gap between wheel and package (e.g. about 10 micrometer spacing gives high levitation force and high torque with low current) and good balance.
It is another object of the invention to provide a microwheel having a magnetic levitation in an evacuated package that is integral to the fabrication process having low weight, high reliability, high efficiency, low cost. It is another object of the invention to provide a microwheel where the wheel and package are of the same material that allows for operation over a wide temperature range and used as multichip module material (MCM) for other electronics.
In summary, the micromachined mesoscopic momentum/flywheel of the invention, called the microwheel herein, can achieve about a tenfold reduction of cost and about a tenfold decrease of weight as compared to the state of the art products. Furthermore, the microwheel enables multifunction integration of altitude control, energy storage, electronic platform, and structural functions and modular and novel redundant architecture.